Glycerol Conversion Catalyzed by Carbons Prepared from

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Glycerol Conversion Catalyzed by Carbons Prepared from Agroindustrial Wastes Maraisa Gonçalves, Victor C. Souza, Thalita S. Galhardo, Michelle Mantovani, Flávia C. A. Figueiredo, Dalmo Mandelli, and Wagner A. Carvalho* Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Rua Santa Adélia, 166, Santo André − SP, CEP 09210-170, Brazil ABSTRACT: Sulfonated carbon-based catalysts were prepared from agroindustrial wastes (sugar cane bagasse, coconut husk, and coffee grounds). These catalysts showed high activity for glycerol etherification with tert-butyl alcohol. Yields of mono-tertbutyl glycerol (MTBG), di-tert-butyl glycerol (DTBG), and tri-tert-butyl-glycerol (TTBG) were higher than that obtained using Amberlyst-15 commercial resin. At 393 K and 5 wt % catalyst loading, glycerol conversion and selectivity to DTBG+TTBG after 4 h reaction time were 80.9% and 21.3%, respectively, with the sugar cane bagasse-based catalyst. Both catalytic activity and selectivity were affected by the presence of water in the reaction medium. However, the flexible and hydrophilic structure of the oxidized carbon allows the adsorption of water without compromising the activity of acid sites.

1. INTRODUCTION Over the past few years there has been increasing concern about global warming, resulting in a growing demand for renewable energy sources, which have significant lower contribution to CO2 emissions. In particular, biodiesel became a world-recognized renewable substitute for fossil diesel. It is generally produced from the transesterification of vegetable oils with alcohols in the presence of an alkaline-based catalyst. This process yields glycerol on a glycerol/biodiesel weight ratio of 1/ 10. With a sharp rise in biodiesel production, an oversupply of glycerol has been created, causing its price to decline substantially. As a consequence, new applications for glycerol are being investigated because it can be considered a promising building block for biorefineries.1 Several processes to produce value-added chemicals from glycerol have been reported, such as hydrogenolysis, etherification, esterification, dehydratation, and oxidation, among others.2−5 Glycerol etherification with tert-butyl alcohol (TBA) is an acid-catalyzed reaction, resulting in a mixture of mono-tertbutyl-glycerol (MTBG), di-tert-butyl-glycerol (DTBG), and tritert-butyl-glycerol (TTBG). Some unwanted by-products can also be formed as a result of polymerization reactions. DTBG and TTBG can be useful as fuel additives or components due to their good blending properties and high cetane numbers. In addition, these oxygenated molecules are able to reduce carbon monoxide and particulate matter emissions from incomplete combustion of the fuel.6 Regrettably, MTBG is the main product obtained in glycerol etherification as a result of the electrophilic attack of a tert-butyl cation (a tertiary carbocation) preferably on the primary carbon of glycerol due to steric hindrance and electrostatic effects exerted by −OH glycerol groups. On the other hand, DTBG and TTBG are preferred as potential oxygenate additives to diesel fuels because of their ease of blending and non-polar properties. Glycerol etherification is reported to be favored in the presence of Brønsted acid catalysts. In general, the most suitable catalysts are commercial resins such as Amberlyst, and © 2013 American Chemical Society

their high activity is caused by the Brønsted acid sites of the sulfonic groups. The major limitation of the use of these resins is their thermal stability (393 K for Amberlyst-15, reaching up to 463 K for Amberlyst-70). Several authors also reported the use of zeolites, heteropolyacids, niobia, and other acidic solids.7−9 Melero et al.10 recently reported the syntheses of two sulfonic-acid-functionalized mesostructured silica catalysts for the etherification reaction. Compared to the conventional resins, these catalysts showed superior activity (total glycerol conversion up to 90%). However, both activity and selectivity losses were observed during consecutive tests for the best catalyst (arene-sulfonic-acid-functionalized mesostructured silica). Additionally, Klepácová et al.7 reported good glycerol conversion (88.7%) and high selectivity to DTBG when a zeolite H−Y was used. Sulfonic acid-functionalized carbonbased materials have also demonstrated an excellent catalytic behavior in some transformations of glycerol. Zhao et al.11 promoted the etherification of isopentene with methanol, catalyzed by a sulfuric acid-treated carbon obtained from glucose and 4-hydroxybenzenesulfonic acid. The authors obtained a conversion of 55.2% at 353 K after 20 h. In this case, glucose was first hydrolyzed to hydroxymethylfurfural, which reacted with 4-hydroxybenzenesulfonic acid to form a phenolic-like resin containing −SO3H groups. Janaun and Ellis12 also tested carbonized sucrose as a catalyst in glycerol etherification, but the conversion was neither determined nor was TTBG detected. Glycerol esterification was investigated by Sanchez et al.13 using a carbon-based catalyst prepared by sulfonation of carbonized sucrose. The results were very interesting; about 99% conversion and high selectivity to triacetylglycerol (50%) were obtained. Liu et al.14 obtained a sulfonated carbon Received: Revised: Accepted: Published: 2832

November 8, 2012 January 15, 2013 January 31, 2013 January 31, 2013 dx.doi.org/10.1021/ie303072d | Ind. Eng. Chem. Res. 2013, 52, 2832−2839

Industrial & Engineering Chemistry Research

Article

samples were filtered and repeatedly washed with warm water until neutrality and then dried in an oven at 393 K for 24 h. Solids were sieved (>60 mesh) and identified as CHC-S, CGCS, and SCC-S. Some experiments were performed controlling the initial moisture content. In this case, the catalysts were kept in an oven at 393 K for variable time intervals, monitoring the humidity of the solids by thermogravimetric analysis. 2.2. Characterization. Textural properties of the different carbons were obtained by nitrogen adsorption measurements at 77 K in an Autosorb-1MP device (Quantachrome Instruments). The “apparent” surface area was estimated according to BET equation based on the adsorption data in the partial pressure range (P/P0) from 0.05 to 0.12. Prior to the measurements, the samples were outgassed at 423 K and 1.3 × 10−3 Pa for 4 h to remove moisture. The surface chemistry of the samples was determined by Fourier transform infrared spectroscopy (FTIR) analysis using a Varian 3100 FT-IR spectrometer. The analyses were performed mixing dried carbon samples with potassium bromide (KBr) in a 1:20 weight ratio and ground into fine powder. This mixture was dried at 373 K for 24 h, and thin pellets were made in manual equipment. The spectra were then acquired at this temperature by accumulating 100 scans at 4 cm−1 resolution in the range of 400−4000 cm−1. The concentration of acidic sites was evaluated using Boehm Titration. For the test, 0.7 g of carbon samples were added in 25 mL of basic solutions prepared previously, NaHCO3 0.1 mol L−1 (Synth), Na2CO3 0.05 mol L−1 (Synth), and NaOH 0.1 mol L−1 (Nuclear). The solutions with carbons were stirred for 72 h at room temperature and filtered prior to titration. Ten milliliters of filtered solution was titrated with HCl 0.1 mol L−1 in an automatic titrator (Metrohm 905 Titrando). The amount of sulfur on the carbon surface was determined by energy dispersive X-ray spectrometry (EDS). Analyses were performed on a JEOL JSM-6701F field emission scanning electron microscope operating at 10.0 kV and 10.0 mA. Thermogravimetric analysis (TGA) was performed in a Q500 TGA device, TA Instruments. Analyses were carried out under N2 atmosphere. In a typical analysis, 10 mg of sample is heated in a platinum pan at 10 K min−1 from 298 to 1073 K in N2 flowing at 100 mL min−1 (STP). X-ray photoelectrons spectra (XPS) were obtained in a VGMicrotech Multilab 3000 spectrometer equipped with a hemispherical electron analyzer using a Mg Kα (1253.6 eV) 300 W X-ray source. The spectra were recorded in the range of 0−1100 eV. The pressure of the analysis chamber was lower than 10−7 Pa, which was increased to approximately 10−5 Pa during ion bombardment. Raman spectra of the SCC-S sample pre-treated with HF (in order to remove the silica) were collected on a Horiba T64000 system using a 532 nm Verdi G5 laser (Coherent Inc.) with intensity of 10 mW. 2.3. Catalytic Tests. Liquid phase etherification reaction between glycerol and tert-butyl alcohol (TBA) was carried out in a 300 mL stainless steel batch reactor with a mechanical stirrer and under autogenous pressure. The system was previously purged 2−3 times with N2, and the experiments were performed under inert atmosphere at 363, 393, and 423 K, with different amounts of catalyst and a TBA:glycerol molar ratio equal to 4:1. Samples were analyzed by gas chromatography (Agilent 7890A, FID, DB-Wax 30 m x 0.25 mm x 0.25 μm) using acetonitrile as the internal standard. The identification of the reaction products was done by GCMS

material by the reaction of 4-benzene-diazonium sulfonate with a commercial activated carbon, using hypophosphorous acid as the reducing agent. The catalyst was tested in the esterification of acetic acid with ethanol, providing a conversion of 78%, while the reaction catalyzed by Amberlyst-15 provided 86%. The authors attributed the lower activity to the fact that the −SO3H groups density in the sulfonated carbon (0.64 mmol g−1) was lower than that found in Amberlyst-15 (4.7 mmol g−1 ). Khayoon and Hameed 5 described a commercial sulfonated activated carbon with high catalytic activity in glycerol acetylation. Ferreira et al.15 promoted the acetylation of glycerol over dodecatungstophosphoric acid supported on activated carbon, obtaining good selectivity to diacetin. The acetalization of 1,3-propanediol biologically derived from glycerol was tested by Boonoun et al.16 using a sulfonated carbon-based catalyst. The catalyst, synthesized by incomplete carbonization of naphthalene in sulfuric acid, was able to promote the reaction of glycerol with acetaldehyde in an aqueous mixture. However, the carbon-based sulfonated catalyst required a longer reaction time and had inferior stability when compared to commercial Dowex 50-WX4−200 and Amberlite IR120 resins. Despite the good results obtained with carbon-based catalysts, the search for new precursors that are cheap, accessible, and with potential to create significant economic valorization is still needed. In Brazil, agroindustrial activity is an important pollutant source, requiring constant research on improving or even proposing new solid waste processing techniques. An alternative use for this kind of solid waste is the production of carbon.17 However, the main application of these materials is as adsorbents, while there are few works devoted to the use of carbon from solid waste as catalyst in glycerol conversion. Typically, these applications involve materials modified by processes that increase their acidity, such as sulfonation. In this work, we propose to add value to two wastes profusely produced in Brazil: glycerol (about 267000 m3 produced in 2012) and some solid wastes from agroindustry, such as sugar cane bagasse, coconut husk, and coffee grounds. Sulfonated carbon prepared from carbonization of agroindustrial wastes and treated with sulfuric acid has been characterized by Fourier transform infrared spectroscopy, nitrogen adsorption, Boehm titration, energy-dispersive X-ray analysis, thermogravimetric analysis, X-ray photoelectrons spectroscopy, and Raman spectroscopy. The solids have been tested as catalyst for etherification of glycerol. The effect of the amount of catalyst, temperature, and initial moisture content, as well as the reusability of the catalysts have also been evaluated and compared to commercial resins Amberlyst-15 and Amberlyst70.

2. MATERIALS AND METHODS 2.1. Synthesis of Catalysts. Three solid wastes were used in the catalysts production: sugar cane bagasse, coconut husk, and coffee grounds. Black carbon was obtained by controlled pyrolysis of the wastes in a tubular furnace under N2 flow (100 mL min−1) at the rate of 10 K min−1 to a final temperature of 673 K, which was kept for 4 h. Black carbon samples were named according to the original waste: coconut husk (CHC), coffee grounds (CGC), and sugar cane bagasse (SCC). Solids were treated with sulfuric acid for surface sulfonation. Treatments were carried out using 100 mL of sulfuric acid and 10 g of each carbon kept under reflux at 453 K for 10 h. All 2833

dx.doi.org/10.1021/ie303072d | Ind. Eng. Chem. Res. 2013, 52, 2832−2839

Industrial & Engineering Chemistry Research

Article

(Shimadzu QP-2010Plus, SGE BP-20-strong Wax 30 m × 0.25 mm × 0.25 μm) and confirmed according to the method described by Jamróz et al.18

sulfonated carbons exhibit an amount of S proportional to the number of acidic groups. SCC-S carbon was also analyzed by XPS and Raman techniques (Figure 1). Peaks at 168.0 and 169.4 eV in the XPS spectra can be assigned to S 2p1/2 and S 2p3/2 photoelectrons from SO3H groups, respectively.20,21 Because there is only one single doublet associated to S, this result suggests that all S atoms are present as −SO3H groups, which is consistent with the literature on the sulfonation of amorphous carbon.22−24 The presence of sulfonic groups was also confirmed by Raman spectroscopy. The spectra shown in Figure 1 exhibit, in addition to the typical D and G-mode bands associated to benzene rings of amorphous carbon (1360 cm−1) and graphene sheets (1600 cm−1), bands at 1742 cm−1 and 1667 cm−1 associated to CO bonds, and at 1060 cm−1 and 600 cm−1 assigned to C−O and skeletal vibrations, respectively, and a large band at 630−790 cm−1 that may be attributed to the C−S vibration.25,26 Infrared spectra obtained for all carbons (not shown here) are very similar. The band at 1710 cm−1 can be assigned to carboxylic group and the other one at 1590 cm−1 to CC stretching. The broad band at 1230 cm−1 can be attributed to esters, ethers, or phenol groups. A characteristic peak of sulfonated groups was observed at 1030 cm−1 in addition to a shoulder at 1750 cm−1. There is some controversy in the literature with respect to the assignment of the desorption temperature of specific surface groups present at the carbon surface. Desorption temperatures are known to be affected by the porous texture of carbon, heating rate, and geometry of the experimental system used. However, some general trends have been established by several authors: a peak resulting from the carboxylic acid group decomposition at low temperatures and at higher temperatures peaks from lactone, phenol, ether, and carbonyl groups decomposition.27 Thermogravimetric analysis (TGA) performed with the original and sulfonated carbons (Figure 2) indicated that the treatment increased the amount of surface groups. SCC-S carbon showed a large peak at 523−723 K attributed to sulfonic, carboxylic, and lactone groups decomposition, and a peak at 823 K attributed to phenolic and carbonyl groups. SCC carbon showed a peak at 373 K attributed to adsorbed water and a peak at 823 K attributed to the decomposition of phenolic and carbonyl groups.28 3.2. Glycerol Etherification. 3.2.1. Comparison of Carbon Sources. A blank reaction carried out with nonsulfonated carbons indicated that the catalytic activity of these carbons was negligible. In order to test the activity and

3. RESULTS AND DISCUSSION 3.1. Catalysts Characterization. Surface areas of all carbons obtained in this work were very small (SBET < 10 m2 g−1) compared to Amberlyst-15 and Amberlyst-70, 53, and 36 m2 g−1 (data provided by the suppliers). These results are expected for these solids, and they are related to the extensive carbonization period and the fact that these carbons have not been subjected to any activation process. Surface area and acidity are very important parameters to be considered when choosing a catalyst; however, the surface acidity seems to be more important in glycerol etherification reactions. In order to analyze carbon surface acidity, Boehm Titration experiments were performed. The amounts of surface groups are presented in Table 1. Table 1. Carbons Surface Groups and Elemental Analysis sample

carboxylic + sulfonic (mmol g−1)

lactone (mmol g−1)

phenolic (mmol g−1)

S (%)

SCC CGC CHC SCC-S CGC-S CHC-S